1. Field of the Invention
The present invention relates to an optical pickup device used in a reading apparatus for an information recording medium such as an optical disk(disc) on which optical information is recorded in the form of tracks and, more particularly, an error detection device and a method therefor which are suitable for a reproduction system being compatible with a DVD (Digital Versatile Disc) and a CD (Compact Disc).
2. Description of the Related Art
Although CDs serving as public optical disk systems are generally popularized, in recent years, DVD systems having higher densities are proposed and merchandised to be popularized. In a DVD player serving as a reproduction apparatus, in order to avoid apparatuses from overlapping or cumbersome use of the apparatus, reproduction being compatible with a CD must be performed. For this reason, a technique for reproducing disks having two types of standards is developed, and simplification and reduction in cost of an arrangement for realizing the technique are demanded.
A tracking scheme(system) is proposed as one of CD/DVD compatible reproduction techniques. In a CD system, a so-called "three-beam method" which detects a tracking error signal by using two sub-beams arranged. before and after a main beam is used. Uneven portions called pits are formed in a CD to record information, and a pit array is called a track. In this three-beam method, two sub-beams are symmetrically arranged with predetermined distances from a main beam, and the two sub-beams are reversely offset by predetermined amounts in a direction perpendicular to the track. When the sub-beams are on the track, average amounts of light decrease. More specifically, when both the sub-beams are offset by equal amounts with respect to a desired track, the main beam is on the desired track. In other words, it is used that the amounts of light of both the sub-beams are equal to each other when the main beam is on the desired track. That is, when the main beam is offset from the track, the amounts of light being incident on two PDs (Photo Detectors) are different from each other. When the main beam is on the track, the amounts of light being incident on the two PDs are equal to each other, and, as a result, a tracking error can be detected.
However, in this method, the following disadvantages are observed. That is, beam positions suitable for two types of disks having different track pitches cannot be easily set, and offsetting easily occurs by a reflected beam from a non-read layer (non-recording layer) in a two-layer disk such as a DVD. For these reasons, in CD/DVD compatible reproduction, a "DPD method" (Differential Phase Detection) using a phase difference formed on four divided portions of a PD (Photo Detector) when the beam traverses the tracks is mainly used (reference: "Development of New Servo Function for DVD Player": Higuchi, Pioneer R & D Vol. 7 No. 1 from pp. 47).
On the other hand, to satisfy the requests for a reduction in cost and miniaturization, an attempt to integrate an optical pickup system is advanced, a device obtained by integrating a semiconductor laser (LD), a PD, and a hologram element (HOE: Holographic Optical Element) is developed. The device has been applied to not only a CD but also a DVD (reference: "Integrated Optical Head for DVD": Mizuno et al., National Technical Report Vol. 43 No. 3 Jun. 1997 pp. 275).
As described in this reference, in an integrated device in which an LD and a PD can be arranged to be close each other, a diffracted beam obtained by an HOE and the light emission point of an LD can be easily arranged at almost conjugate positions. Focus error detection by a complementary "SSD method" (Spot Size Detection) using .+-.1-order(.+-.1st-order) diffracted beams obtained by the HOE can be realized.
The principle of the SSD method is shown in FIGS. 1A to 3C. FIG. 1A is a sectional view showing the relationship between a hologram substrate 117, five-piece light-receiving elements 102 and 103, and a light emission point 101, and FIG. 1B is a plan view showing the two five-piece light-receiving elements 102 and 103 arranged on a silicon substrate and the position of an LD serving as the light emission point 101. A grating pattern 106 is arranged on the lower surface of the hologram substrate 117, and a hologram pattern 105 is arranged on the upper surface. A laser beam 104 emitted from the light emission point 101 is diffracted by the grating pattern 106 on the lower surface of the hologram substrate 117 to be three beams, i.e., a 0-order(0th-order) beam (main beam) and .+-.1-order diffracted beams. The three beams are focused on a disk (not shown) arranged above an objective lens (not shown) through the objective lens arranged above the hologram substrate 117. The reflected beams from the disk pass through the objective lens to be diffracted by the hologram pattern 105 on the upper surface of the hologram substrate 117. The diffracted .+-.1-order beams are received by the two five-piece light-receiving elements 102 and 103. Although each of the five-piece light-receiving elements 102 and 103 is divided by five, three central PD's(Photo Detectors) PD1, PD2, and PD3 (see FIGS. 2A to 2C) and three central PD's(Photo Detectors) PD4, PD5, and PD6(see FIGS.3A to 3C) of the five-piece light-receiving elements 102 and 103 are used in the SSD. The two remaining PDs on both the sides of each five-piece light-receiving element are used in the tracking scheme of the three-beam method.
FIG. 1A illustrates a case wherein the +1-order(+1st-order) diffracted beam diffracted by the hologram pattern 105 is focused in front of the five-piece ligh-treceiving element 102, the -1-order(-1st-order) diffracted beam is subjected to a divergent effect and focused after the -1-order diffracted beam passes through the light-receiving surface of the five-piece light-receiving element 103. FIGS. 2A to 2C show spot sizes which change depending on the focal positions of the three central PD's PD1, PD2, and PD3 of the five-piece light-receiving element 102, and FIGS. 3A to 3C show spot sizes which change depending on the focal positions of the three central PD's PD4, PD5, and PD6. FIGS. 2A and 3A show the spot sizes obtained in an in-focus state, FIGS. 2B and 3B show the spot sizes obtained when the disk is distant, and FIGS. 2C and 3C show the spot sizes obtained when the disk is close. As shown in FIGS. 2B and 3B, when the disk is distant from the objective lens, the spot size of the +1-order diffracted beam is large, and the spot size of the -1-order diffracted beam is small. In contrast to this, as shown in FIGS. 2C and 3C, when the disk is close to the objective lens, the spot size of the +1-order diffracted beam is small, and the spot size of the -1-order diffracted beam is large. Therefore, when the difference between a total of outputs PD1+PD2+PD3 from the PD1, PD2, and PD3 and a total of outputs PD4+PD5+PD6 from the PD4, PD5, and PD6 is calculated, focus error detection by the complementary SSD method. The focus error detection method by the complementary SSD method has the following advantages in comparison with a "knife edge method" which is also realized. That is, the position of the HOE is not necessarily exactly adjusted, one of the .+-.1-order diffracted beams need not be rejected, high efficiency can be obtained.
As described above, when the error detection method and the prior art of the embodiment of the error detection method are considered, in focus error detection by the "knife edge method", the positional relationship between an HOE diffracted beam and a PD division line must be exactly set. Even in the assembly process of only the device, adjustment using a disk and a reproduced signal as an index must be performed, and large-scale equipment and processes are required. In addition, it is difficult to actually obtain the arrangement using both the .+-.1-order diffracted beams, and only one of the diffracted beams is used. For this reason, the efficiency is low, and an increase in load on the LD, degradation of signal quality, or the like are caused. Furthermore, when the "DPD method" is to be combined to the tracking error detection by the method, the diffracted beam is divided in two. The half is used in focus error detection, and the other half is used in tracking error detection. For this reason, the efficiency is degraded, and, in the "DPD method" in which four divided regions must be originally used, two regions are used in place of the four divided regions. Therefore, offset correction of a tracking error cannot be easily performed.
In comparison with this method, the "SSD method" is used in focus error detection in the integrated device described in the reference ("Integrated Optical Head for DVD"). The problems described above are partially improved by combining the focus error detection to the tracking error detection by the "DPD method". More specifically, position adjustment can be omitted, and degradation of efficiency is prevented by both the .+-.1-order diffracted beams of HOE. However, design is made to use one of the .+-.1-order diffracted beams for only a focus error and the other for only a tracking error. One of the .+-.1-order diffracted beams is rejected for each error signal. This is disadvantageous in efficiency. Even if the dividing method for light-receiving regions without changing the design, a focus error signal and a tracking error signal cannot be detected by both the diffracted beams.